How to use ALSA LADSPA plugin?

I’m trying to use a system wide DSP directly on top of ALSA. Here’s my .asoundrc:

pcm.DAC {
	type hw
	card 1
	device 0
}

pcm.DSP {
	type plug
	slave {
		format FLOAT
		rate "unchanged"
		channels "unchanged"
		pcm {
			type ladspa
			path "/usr/lib/ladspa"
			playback_plugins [{
				label "ladspa_dsp"
			}]
			slave.pcm {
				type plug
				slave {
					pcm "DAC"
					rate "unchanged"
					channels "unchanged"
				}
			}
		}
	}
}

pcm.!default {
	type copy
	slave.pcm "DSP"
}

ctl.!default {
	type hw
	card "DAC"
}

I can’t find a way to get QuodLibet to use “DSP” over “DAC”. I can’t put anything besides alsasink device=DAC or it errors out. Also, the error shows up only once; after that, there’s no more error dialogs even with an invalid argument to alsasink.

I understand that this might be more related to ALSA/GStreamer than QuodLibet, but there is not much documentation on the pipeline options either, or if there are, they’re not novice-friendly.

How would I go about feeding bit-perfect playback into the DSP and have QuodLibet use that as output?